Abstract: This paper analyzes the principle of switching power supplies and elucidates the working principle of each module inside a DC-DC power management chip. It proposes a design concept, explains in detail the working principle of the functional modules, and finally implements the chip using BiCMOS technology.
Keywords: Switching power supply , current-mode PWM control, boost converter
introduction
The power supply is the heart of all electronic devices, and its quality directly affects the reliability of these devices. This is especially true for switching power supplies, which are receiving increasing attention. Current computer equipment and various high-efficiency portable electronic products are trending towards miniaturization, resulting in relatively high power consumption. This necessitates smaller, lighter, and more efficient battery-powered systems, requiring the use of high-efficiency DC/DC switching power supplies.
The current development trend of power electronics and circuits is mainly towards modularization and integration. Dedicated chips with various control functions have developed rapidly in recent years. Integration and modularization have made power supply products smaller and more reliable, greatly facilitating applications.
On the other hand, in switching power supply DC-DC converters, fluctuations in input voltage or output load are possible. Maintaining the average DC output voltage within the required amplitude deviation range requires complex control techniques, making the study of various PWM control structures a hot research topic. Under these circumstances, designing and developing switching power supply DC-DC control chips is valuable both economically and scientifically.
1. Analysis of the Principle of Switching Power Supply Control Circuit
A DC-DC converter uses the switching of one or more switching devices to transform a DC input voltage of one level into a DC output voltage of another level. One method to control the average output voltage under a given DC input voltage is to use a fixed frequency for switching and adjust the conduction interval length to control the average output voltage; this method is also known as pulse width modulation (PWM).
PWM can be divided into two categories based on its control method: voltage-mode control and current-mode control. The basic principle of voltage-mode control is to compare the output signal of an error amplifier with a fixed sawtooth wave to generate the PWM signal for control. From a control theory perspective, voltage-mode control is a single-loop control system. A voltage-controlled converter is a second-order system with two state variables: the voltage across the output filter capacitor and the current through the output filter inductor. A second-order system is conditionally stable; only through careful design and calculation of the control circuit, and under certain conditions, can the closed-loop system operate stably. Figure 1 shows the block diagram of voltage-mode control.
Figure 1. Block diagram of voltage-type control principle
Current-mode control compares the output signal of the error amplifier with the sampled peak inductor current, thereby controlling the duty cycle of the output pulse so that the peak inductor current changes with the error voltage. Current-mode control is a first-order system, and first-order systems are unconditionally stable systems. It adds a current negative feedback loop to the traditional PWM voltage control, making it a dual-loop control system. This transforms the inductor current from an independent variable, turning the second-order model of the switching converter into a first-order system. As shown in Figure 2, compared to a single closed-loop voltage control mode, current-mode control is a dual-loop control system. The outer loop is formed by the output voltage feedback circuit, and the inner loop is formed by the current transformer sampling the output inductor current. In this dual-loop control, the voltage outer loop controls the current inner loop; that is, the inner loop current rises in each switching cycle until it reaches the error voltage threshold set by the voltage outer loop. The current inner loop performs instantaneous, rapid pulse-by-pulse comparisons and monitors the dynamic changes of the output inductor current, while the voltage outer loop only controls the output voltage. Therefore, the current-mode control has a much larger bandwidth than the voltage-mode control.
Figure 2 Block diagram of current-mode control principle
Current-mode control offers several advantages: excellent linear regulation (voltage regulation); the entire feedback circuit becomes a first-order circuit, simplifying the error amplifier's control loop compensation network, improving stability and frequency response, and providing a larger gain-bandwidth product; instantaneous peak current limiting; and simplified design of feedback control compensation networks, load current limiting, and flux balancing circuits, reducing the number of components and cost. This is significant for improving the power density of switching power supplies and achieving miniaturization and modularization. However, it also has disadvantages. For example, system instability may occur when the duty cycle is greater than 50%, potentially generating subharmonic oscillations. Furthermore, there are limitations in circuit topology selection; in boost and buck-boost circuits, the energy storage inductor is not located at the output, leading to errors between peak and average current. It is also sensitive to noise and has poor noise immunity. Solutions to these disadvantages exist, with ramp compensation being a necessary method.
2. Design of internal modules of the chip
The objective of this study is to design a boost DC-DC power converter chip based on PWM control. This chip implements a current-mode PWM control circuit based on a first-order dual-loop (voltage loop and current loop) control system. The integrated module will include control, drive, protection, and detection circuits. Finally, based on the basic framework of the circuit system, and combining power electronics and microelectronics technologies, the study focuses on the implementation of the DC-DC conversion circuit using BiCMOS technology.
System design, system block diagram, and design concepts for each functional module
Figure 3 System module principle block diagram
The following sections describe each functional module of the system:
① The error amplifier circuit is a high-gain differential amplifier used to adjust the converter. The amplifier generates an error signal, which is supplied to the PWM comparator. The error signal is generated when the output voltage sample is compared with an internal voltage reference and the difference is amplified. Pin 2 (Vref) of the error amplifier is the fixed reference voltage generated by the reference voltage.
②When the PWM comparator receives the current sampling signal, which is the current signal after adding the inductor current and the compensation harmonics generated by the oscillator, if it exceeds the error signal, the PWM comparator flips, and the reset drive latch disconnects the power switch, thereby controlling the switching transistor to turn on and off.
③ Oscillator Module: The oscillator circuit provides a clock signal of a certain frequency to set the converter's operating frequency, as well as a timing ramp wave for slope compensation. The clock waveform is a pulse, and the timing ramp wave is used for ramp compensation, and they are added at the inductor sampling terminal.
④ Driver Latch: The latch includes an RS flip-flop and related logic, which controls the state of the power switch by turning the drive circuit on and off. A low output level from the latch turns it off. In normal operation, the flip-flop is set high during the clock pulse, and the latch is reset when the PWM comparator output goes high.
⑤ Soft-start circuit module: When the entire system is first started, the inductor generates a large inrush current. Soft-start prevents the system from starting at full duty cycle, allowing the output voltage to increase to the rated regulation point at a controlled rate. The design idea is to use the charging and discharging of an external capacitor to gradually increase the duty cycle, thereby achieving output stability.
⑥ The current sampling circuit provides slope-compensated current-sensitive voltage to the PWM comparator.
⑦ The protection circuit module monitors the current of the power switch. If the value exceeds the rated peak value, the circuit will activate and restart the soft start cycle.
3. Several details that must be considered in the design
①Regarding oblique wave compensation
This is a fundamental problem in the current-controlled switching converters mentioned above. Current-controlled converters compare the actual inductor current and the current value set in the outer voltage loop across a PWM comparator to control the switching transistor. The reason for ramp compensation is analyzed below. The following figures show the inductor current waveforms controlled by peak current with duty cycles greater than 50% and less than 50%, respectively.
Figure 4. Analysis of the slope compensation principle
Where Ve is the current setpoint of the voltage amplifier output, ΔI0 is the disturbance current, and m1 and m2 are the rising and falling edge slopes of the inductor current, respectively. As shown in the figure, when the duty cycle is less than 50%, the current error ΔIl caused by the disturbance current decreases, while when the duty cycle is greater than 50%, the current error ΔIl caused by the disturbance current increases. Therefore, in peak current mode control with a duty cycle greater than 50%, the disturbance signal will amplify after one cycle, causing instability. In this case, slope compensation needs to be added to the comparator to stabilize the circuit. With slope compensation, even if the duty cycle is less than 50%, the circuit performance can still be improved. Therefore, slope compensation can effectively increase circuit stability, prevent the average inductor current from changing with the duty cycle, and reduce the error between the peak and average values. Slope compensation can also suppress subharmonic oscillations and ringing inductor current. Details will not be elaborated here. For slope compensation, it is essential to determine the precise magnitude of the slope of the compensation waveform. The method used is to establish a system model, derive the transfer function, and calculate the value of the compensation slope. This is a crucial step.
② Regarding the soft start issue
DC/DC switching power supplies are prone to inrush current during startup, which can damage the electronic system. To avoid excessive input current and output voltage overshoot during startup, a soft-start circuit must be used in the design. However, this method has drawbacks: if the output voltage threshold is not reached, inrush current may occur, potentially damaging the electronic system; and even after the output voltage reaches the threshold, accidental overcurrent may cause the power supply to restart multiple times. Therefore, a period-to-period current limiting threshold should be used to limit the inrush current during power-up and prevent multiple restarts of the power supply. (See Figure 5)
Figure 5 Soft start circuit
4. Summary
This paper provides a detailed analysis of the working principle of switching power supplies, designs the internal modules of the chip, and finally implements the chip using BiCMOS technology. It offers a holistic understanding of chip system design and elaborates on the underlying design principles. This approach is highly helpful for chip system design in other fields and is therefore of great significance.
References
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